1. Field of the Invention
The present invention provides a water-based ink, more particularly a water-based ink that can provide an image having high image quality, excellent rub-resistance and excellent fastness, especially water fastness, and high reliability. The present invention also provides a recorded image formation method and an image recorded with the above ink. As a preferable application, it also provides a water-based ink excellent in fluorescent emission properties, and a recorded image formation method and a recorded image using the ink.
2. Related Background Art
Various inks have been proposed and studied for water-based ink including writing inks and ink-jet recording inks, aiming at improvement of image fastness and image quality.
Conventionally, water-soluble coloring materials are used in water-based inks, so that the inks are excellent in reliability such as sticking resistance and stability with time, and can provide high image density. On the other hand, image fastness such as water resistance and light resistance of the recorded images is often unsatisfactory.
As a countermeasure, for example, use of a water-soluble coloring material having carboxylic acid has been proposed. However, while water resistance of the image is improved, the coloring material in the ink tends to precipitate and stick, which causes clogging if the ink is used in an ink-jet recording method in which ink droplets are discharged through a nozzle.
Recently, use of water dispersing coloring materials such as carbon black and organic pigments in water-based ink has been proposed to improve image fastness such as water resistance and light resistance. In a popular water-based ink using a water dispersible coloring material, a coloring material is dispersed in a water system with a chemical or physical action of a surfactant or resin. When the ink is attached to a recording material, water and liquid media evaporate on the recording material, penetrate and diffuse into the recording material, thus causing aggregation of the water dispersible coloring material to improve image fastness, e.g. water resistance and light resistance, of the recorded image. On the other hand, in recording with an ink containing a water-dispersible coloring material, all of the coloring material aggregates on the surface of a recording material, resulting in poor rub-resistance of the recorded matter.
Meanwhile, Japanese Patent Application Laid-Open No. 2000-113331 discloses an invention for improving fluorescent properties in a relationship between a solvent and a fluorescent coloring material. This publication discloses a recording ink having two types of fluorescent coloring materials of the same color (addition of coloring materials having no fluorescence is also described in Examples), and containing mutually incompatible two types of organic solvents (such as glycerin and a nonionic surfactant) and pure water dissolving them.
In any case, a conventional pigment (water-dispersible coloring material) ink contains a relatively large amount of pigment (e.g. 4% by mass), a surfactant at a level for improving permeability (e.g. 1% by mass), and the like, and the recorded image with such an ink comprises the coloring material aggregated as a plane on the surface of the recording medium.